Fighters of the Islamic State of Iraq and the Levant (ISIL) celebrate on vehicles taken from Iraqi security forces, at a street in city of Mosul.Reuters

Iraq's most senior Shia cleric has called on the people of Iraq to take up arms and defend the country against the Islamic State of Iraq and the Levant (Isis).

"Citizens who are able to bear arms and fight terrorists, defending their country and their people and their holy places, should volunteer and join the security forces to achieve this holy purpose," Grand Ayatollah Ali al-Sistani's representative Sheik Abdulmehdi al-Karbalai announced during Friday prayers in the city of Karbala.

Al-Sistani, the highest ranking Shia authority in Iraq, made the call to arms as Islamist militants continue to advance on Baghdad.

Security and local officials have reported clashes between Sunni insurgents and Shia militia at two different locations in the eastern province of Diyala.

The country's southern regions are dominated by a Shia Muslim majority whom Isis regard as "infidels".

Al-Sistani had earlier called for all of the country's political factions to unite against the insurgency in a statement released which condemned the capture of Nineveh province.

A United Nations spokesman has said that the Islamist group carried out summary executions of civilians and members of the Iraqi army in the northern city of Mosul which could run into the hundreds.

Militants captured Mosul before heading south, reaching the town of Samarra and the eastern region of Diyala.Google Maps

Iraq has begun to boost defences around the capital of Baghdad as militants edge closer to the heart of the country.

"We put in place a new plan to protect Baghdad," Brigadier General Saad Maan told AFP.

"The plan consists of intensifying the deployment of forces, and increasing intelligence efforts and the use of technology such as (observation) balloons and cameras and other equipment," Maan said.

The US and Iran have both pledged to assist Iraqi prime minister Nouri al-Maliki in his battle against the insurgency.

Iran have reportedly deployed two Revolutionary Guard units to Iraq while US President Barack Obama has said that he is looking at "all options" to help Iraqi prime minister Nouri al-Maliki thwart the militants.

"I don't rule out anything because we do have a stake in making sure that these jihadists are not getting a permanent foothold in either Iraq or Syria," Obama said.

Shia Militias Join the Battle

Sunni insurgents fought Iraqi Shi'ite militia at two locations in Diyala province, a security source and a said.

The clashes took place in Udhaim, north of Baghdad and in Muqdadiya, northeast of the capital.

Meanwhile, thousands of Shia fighters have rushed to the central Iraqi city of Samarra to defend two shrines that were blown up by insurgents eight years ago, sparking the sectarian war that almost destroyed the country.

Convoys of fighters were seen being escorted north by Iraqi police trucks from Baghdad y and many have now reached the city where insurgents were in control after a lightning strike south.

The volunteer Shia fighters were quickly assembled after Iraqi forces abandoned their positions in most of the area, leaving only a small number of troops to guard the Imam al-Askarien shrines.

Samarra is the fourth northern city to have all but fallen out of government control. The embattled prime minister, Nouri al-Maliki, appears to have drawn battle lines further south in Taiji, hoping to defend Baghdad against insurgents who have occupied the north virtually unopposed.